The present invention relates to a method for defining the characteristics of metal electrodes of ceramic sensor elements, where the metal electrodes are deposited as layers and subjected to a subsequent annealing process, and the electrode layers to be defined are placed in the magnetic field of an a.c. current-carrying measuring coil, which detects the electrical and, if indicated, the magnetic properties of the layers to be evaluated, in the magnetic field, and analyzes them with respect to specific criteria, and to a device for implementing the method.
A method of this kind discussed in German Published Patent Application No. DE 196 52 750 A is already being successfully used in the batch testing of a galvanically chromium-plated cassette for a valve device. Using an eddy-current measuring method, one can determine the thickness of a layer of electrically conductive material, in this case of a chromium layer, for purposes of manufacturing control.
In the manufacturing of a ceramic hydrocarbon sensor of the firm Robert Bosch GmbH, the outer electrode is fabricated by a subsequent galvanic gold plating of a sintered Pt cermet electrode located underneath a porous protective layer (see German Published Patent Application No. DE 198 33 087 A). In this electroplating step, approximately 1 to 2 mg of atomic gold are deposited within the porous protective layer and subsequently sintered. The quantity of the deposited gold and the manner in which it is distributed during the sintering process substantially determine the functional properties of the sensor.
The electroplating process can be checked using a non-destructive measuring method, such as analytical balancing (0.1 mg accuracy), current-time measurement (coulomb metric measurement) during electroplating, which is only possible given a high current gain with respect to the Au3+-reduction, or through electrolytic testing using a peroxo compound.
The possible measuring methods named above are either not precise enough, not adequately developed, or encumbered with disadvantages.
The object of the present invention is to render possible a non-destructive, simple and economical method, capable of being automated, for defining the characteristics of metal electrodes of ceramic sensor elements, where the electrodes are deposited in layers and subjected to a subsequent annealing process, which will enable the distribution and quantity of the deposited metal to be controlled when manufacturing ceramic sensor elements of this kind.
In the case of the method proposed here, the deposited Au0-quantity (that is not accessible in the protective layer) is indirectly determined. This is accomplished by a layer-thickness determination in a before/after comparison with the aid of an eddy-current measuring method.
The method according to the present invention makes it possible to distinguish among uncoated, coated, and annealed sensor ceramics. As a result, an acceptance test can be automatically performed on the electroplating process in a series production, and the deposited gold quantity can be indirectly determined.
Assuming sufficient accuracy, the test value also permits direct inferences to be made with respect to the sensor function/characteristics state, which, after all, depend directly on the quantity and distribution of the deposited gold.
In the case of gas sensors having a complex design, such as an NOx dual-chamber sensor, given a high-level of accuracy of the method according to the present invention, there is the possibility of using specific frequencies to detect and test deeper electrode layers.
The entire measuring sequence carried out using the method according to the present invention is simple, fast, and capable of being fully automated.